In the area of automotive technology, including hybrid vehicles, vehicles with hydraulic systems for vehicle components include a mechanical connection to an engine. For example, vehicles with automatic transmissions rely on engine rotation, at a minimum speed, to provide power to an internal pump of the transmission to provide hydraulic pressure to transmission clutches, and additionally to cool transmission components. In this system the engine is mechanically connected to the transmission.
Referring to FIG. 1, a prior art power system 10 for a hybrid vehicle 60 includes an engine 12 which is connected to a torque converter 14 which in turn is connected to an automatic transmission 16. The engine 12 is connected to the torque converter 14 and the transmission 16, and the transmission pump 18 via mechanical linkage 15. An electronic control module (ECM) 20 communicates with the automatic transmission 16 for electrically controlling the automatic transmission 16. The ECM also may communicate with other system of the vehicle, alone or in combination, for example, the engine, automatic braking system (ABS), power take off (PTO), and body controller. The ECM 20 may be powered by, the vehicle low voltage electrical system (battery and/or alternator) and/or electrical system 50 of a hybrid powered vehicle 60. A transmission pump 18 is located within the transmission 16. The internal transmission pump 18 supplies the automatic transmission 16 and the torque converter 14 with transmission fluid flow and maintains fluid pressure. The internal transmission pump 18 is coupled to the power steering pump 22 as the engine may drive both the internal transmission pump 18 and the power steering pump 22 as designated by linkage 15. The power steering pump 22 may be connected via a hydraulic hose 24 (alternatively a high pressure hose may be used) for hydraulic fluid transfer, to a steering fluid reservoir 30. The power steering pump 22 is also connected to a steering gear 26 via the hydraulic hose 24. Likewise the hydraulic hose 24 connects the steering gear 26 to the steering fluid reservoir 30. The steering gear 26 and pump 22 are connected to, and dependent on, the engine for engine rotation to drive the pump 22, for example, one or more belts to drive the pump using engine rotation. A disadvantage of the above hydraulic system is that the engine 12 needs to rotate in order for the hydraulic pumps to function. Also, the engine may need to maintain a threshold speed (revolutions per minute (RPM)) to maintain adequate fluid pressure in a hydraulic circuit, or the engine may need to maintain a higher RPM during maximum load in a hydraulic circuit. Thus, in the system 10, the automatic transmission 16 relies on engine rotation to provide power to the internal transmission pump 18 to provide hydraulic pressure to its clutches and additionally to lubricate and cool transmission components. Similarly, the system 10 uses engine rotation to provide power to the power steering pump 22.
Other vehicle hydraulic systems may use an electric hydraulic pump for supplying transmission fluid. In one instance, a vehicle hydraulic system may include two hydraulic pumps, one mechanically coupled to an engine, and the second electrically coupled to a source of electric energy, wherein both pumps provide fluid pressure to the transmission. In another example, a vehicle hydraulic system may include a pump powered either by electric motor or engine for providing fluid pressure to the transmission only. However, one disadvantage of the above systems is that they include mechanical connection to the engine, as they rely on engine rotation to power the pump. Another disadvantage with known systems includes that one or more pumps are used for each vehicle component, for example, the transmission, and the pump is typically located inside the transmission housing which may be difficult to access when servicing the pump.
Therefore, a need exists for a hydraulic system that can provide fluid pressure to a hydraulic circuit when an engine is idling at a low RPM or off. It would further be advantageous for the hydraulic system to provide the above while increasing the ease of maintenance of the hydraulic pump.